Method and apparatus for security in a wireless network

ABSTRACT

The present invention relates to an apparatus and method for preventing unauthorized users from gaining access to a wireless network. A processor allows only data transmissions from the directions of the authorized users to access the network. Data transmissions from other directions are not allowed to access the network. The present invention also relates to a method and apparatus fore transmitting information only in selected directions, while in directions not selected, information cannot be inferred from the transmitted signal or signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/067,441, filed on Feb. 25, 2005 which is now U.S. Pat. No. 7,398,078,which claims the benefit of U.S. Provisional Application No. 60/550,355,filed Mar. 5, 2004, U.S. Provisional Application No. 60/550,411, filedMar. 5, 2004, and U.S. Provisional Application No. 60/561,433, filedApr. 12, 2004, each of which is hereby incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

Wireless networks bring voice and data communications to both mobile andstationary users. The present invention is particularly suited to twotypes of wireless networks. The first type of wireless network is calleda wireless local area network (WLAN or Hotspot) where data and voicecommunications are provided within a building or within close proximityof a building. In a WLAN, users typically employ laptop computers withantennas that communicate with wall-mounted antennas connected to awired network. The second type of wireless network is called a wirelessmetropolitan area network (WMAN) where data and voice communications areprovided to residential and business premises via radio links thatconnect to the Internet backbone. The radio links are typically obtainedby mounting antennas on houses and lampposts. Additionally, the presentinvention may be well suited for other types of wireless networks.

In wireless networks, the data signals are transmitted through air, sothere is no shield protecting transmitted signals from eavesdroppers.For this reason, it is more difficult to create a secure wirelessnetwork than a secure wired network. Unauthorized users of a wirelessnetwork may potentially gain access to the network to actively stealinformation and change network parameters.

Phased Antenna Arrays

A phased array antenna consists of multiple antenna elements that arefed coherently to produce desired radiation patterns. For example, ifthe array elements, as shown in FIG. 1, are z-directed dipoles that arehalf a wavelength apart and all fed by the same signal, the combinedradiation pattern will have a sharp peak in the direction parallel tothe y-axis (the broadside direction). Alternatively, by applyingdifferent time delays to the signals that feed each dipole, theradiation pattern can be made to peak in the direction of the x-axis(the endfire direction). Thus, the radiation characteristics of theantenna array can be changed electronically without changing thephysical configuration of the array.

A phased array antenna can also be used as a receiving antenna in whichthe elements receive the signals from the transmitters. The outputs ofthe elements are time shifted and combined to achieve a desired arrayreceiving pattern. For example, if the element outputs of the array inFIG. 1 are added in phase, the array receiving pattern will have a sharppeak in the direction parallel to the y-axis (the broadside direction).In this configuration, the array will be most sensitive to signalsbroadcast by transmitters that are located on the y axis. Alternatively,by applying different time delays to the element outputs, the receivingpattern can be made to peak in the direction of the x-axis (the endfiredirection). Thus, the receiving characteristics of the antenna array canbe changed without changing the physical configuration of the array. Theability to electronically modify both transmitting and receivingcharacteristics makes phased arrays particularly useful for radar andcommunications applications.

The following books describe the theory and design of phased arrays: (1)R. C. Hansen, “Phased Array Antennas,” John Wiley & Sons, 1998, (2) R.J. Mailloux, “Phased Array Antenna Handbook,” Artech House, 1994, and(3) R. S. Elliot, “Antenna Theory and Design,” IEEE Press, 2003.

Consider the 18-element array shown in FIG. 1 with element spacing equalto half of a wavelength. A typical sum pattern for this array is shownin FIG. 2. The sum pattern has a main beam and a number of side lobes.The sum pattern is used in radar applications to detect a target. Themain beam is not narrow enough, however, to determine a precise locationof the target. Antenna arrays in communication systems use sum patternsto transmit and receive data in a given direction.

FIG. 3 shows a typical difference pattern for the array shown in FIG. 1.The difference pattern has a sharp null in the main beam direction andside lobes similar to the sum pattern. The sharp null can be used inradar systems to determine the precise location of a target after it hasbeen detected by the sum pattern. This is achieved by steering thedifference pattern to the direction where the target return is exactlyat the null. Difference patterns are also used to suppress jamming byplacing nulls at the directions from which the jamming signals emerge.

With adaptive phased arrays, also known as smart antennas, the receivedsignals and environmental parameters are fed to powerful processors thatsteer the beams to optimize performance. The technology for designingand constructing adaptive phased arrays with hundreds of elements thatproduce prescribed sum and difference patterns has reached a maturestage, as described in the following books: (1) M. I. Skolnik, “RadarHandbook,” McGraw-Hill, 1990, 2^(nd) edition, (2) R. T. Compton,“Adaptive Antennas,” Prentice-Hall, 1998, and (3) G. V. Tsoulos, ed.“Adaptive Antennas for Wireless Communications,” IEEE Press, 2001.

Wireless Communications

Vivato Inc. and ArrayComm Inc. have implemented adaptive phased arrayantennas systems that enhance the performance of wireless communicationsystems. These systems are commercially available at the present time.

Vivato Inc. uses smart antennas. According to Vivato publications,Vivato technology implements a phased-array antenna to create narrowbeams of “wireless fidelity” (“Wi-Fi”) transmissions that are directedto clients on a packet-by-packet basis. Using a technology Vivato refersto as PacketSteering™, a Wi-Fi beam is formed for the duration of apacket transmission. When transmitting data, rather than transmit in alldirections, the switches transmit narrow Wi-Fi beams anywhere within a100 degree field of view. The result is that the switch concentrates RFenergy into a narrow beam, which allows Wi-Fi switches to extend therange of Wi-Fi, typically only tens of meters, to kilometers. While thedistance range is extended, the switch directs radio energy at specificclients within a narrow beam. Additionally, the directional nature ofthe transmissions reduces interference.

Vivato Switches enable parallel operations to numerous users bycommunicating on three non-overlapping channels simultaneously. Theswitch capacity is flexible and can be used when and where it is neededbecause it can communicate with all of the devices within the wide fieldof view. Vivato Wi-Fi Switches support and communicate directly withclient devices based on the 802.11b standard.

ArrayComm IntelliCell™ technology is directed to interference managementand signal quality enhancement using antenna arrays. A typical basestation uses a single antenna or pair of antennas to communicate withthe users, but a base station equipped with ArrayComm IntelliCell™technology employs an antenna array with sophisticated signal processingto reduce the amount of excess energy radiated by the base station.Simultaneously, the signal processing allows the base station to respondselectively to users, mitigating the effects of interference introducedby other network users. The ArrayComm antenna array also provides a gainin signal power, improving the radio link quality while using the sameamount of total power radiated by the base station and user terminal.Improved link quality translates into higher data rates, extended range,and longer battery lifetimes at the user terminals. With IntelliCell™technology, each cell in a network can use the same frequency allocationby eliminating inter-cell interference. Additionally, ArrayCommtechnology enables a system to reuse a frequency allocation within agiven cell by directing energy only where it is required. IntelliCell™technology uses an antenna array to increase the capacity of cellularnetworks by factors of from 3 to 40.

ArrayComm Inc. recently participated in a test described in an articleentitled “iBurst System Showcased in Latest Broadband Wireless Demo”from Broadband Wireless Exchange Magazine, published by BroadbandWireless Exchange. (The article is available atwww.bbwexchange.com/publications/newswires/page 546-638770.asp.)Further, the founder of ArrayComm published an article in ScientificAmerican that describes the capabilities of modern array antennasystems. (M. Cooper, “Antennas Get Smart,” Scientific American, pp.49-55, July 2003).

Security Features in Wireless Communications

Currently commercially available wireless communication systems rely onone or more of the following types of security features: encryption,authentication, scanning and monitoring to detect unauthorizedtransmissions, highly directive antennas, and placement of nulls inreceiving patterns.

Each of these security approaches has flaws. The article “The keyvanishes,” published in the New York Times, Feb. 20, 2001, describes howeven “unbreakable” encryption codes can be overcome. Peter G. Neumann,SRI, security expert, is quoted in this article as saying, “If you thinkcryptography is the answer to your problem, then you don't know whatyour problem is.”

Authentication cannot prevent an unauthorized user from getting accessto the network if that user steals the identification parameters of anauthorized user by eavesdropping. Similarly, with scanning andmonitoring one cannot detect an unauthorized user if that user hasstolen the identification parameters of an authorized user.

Highly directive antennas radiate narrow beams that have very low valuesoutside a main-beam direction, but such narrow beam antennas stillradiate intelligible signals in all directions that may be understood byreceivers with high gains. One example of a narrow-beam radiationpattern is shown in FIG. 2. However, even when the receiving array ishighly directive, an unauthorized user can still gain access to thenetwork by broadcasting high-energy signals. This will be demonstratedbelow in an example.

Placement of nulls in receiving patterns is an effective way of blockingan unauthorized transmitter that is located at a known point in the farfield of the receiving array. However, if the unauthorized transmittermoves around, the receiving array has to track the transmitter andcontinuously modify its receiving pattern. This is a complex task andmay not be practical for wireless communication systems. Moreover, thenulling approach does not work well if the unauthorized transmitter isin the near field of the receiving array. Mailloux describes a furtherlimitation of nulling: “An N-element array can have up to (N−1) nulls,and in principle can cancel up to (N−1) interfering signals. Inpractice, one cannot place too many of the nulls close together withoutincurring severe pattern distortion.” (Mailloux, page 170.) Hence, ifthe receiving array has only a few elements, one is capable of cancelingonly a few unauthorized transmitters.

One article related to wireless network security issues and marketdemands for better security, instead of more security products, isSecurity is the #1 WLAN [Wireless LAN] deployment barrier by Mike Klein,CEO Interlink Networks. (This article is available atwww.intel.com/capital/cases/wifi_infrastructure.htm#Interlink.)

The present invention overcomes the aforementioned problems and providesa physical layer of security for wireless communications that makes thetransmitted signals unintelligible in all but the selected direction ordirections. The physical layer of security makes the wireless networksmore like wired networks and works in conjunction with existing securitymeasures.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a method for securely transmittingdata over radio waves comprising the steps of transmitting one or moredata beams comprising the data with a radiation pattern, transmittingone or more scramble beams comprising one or more signals from whichdata cannot be inferred and one or more radiation patterns, andadjusting the data beams and the scramble beams such that the scramblebeams overshadow the data beams in all but selected directions. Ascramble beam can contain a separate intelligible data stream intendedfor receivers located in the directions where that scramble beamovershadows all other beams.

In one embodiment of the present invention, the scramble beam containsan intelligible data stream that is used to convey information, fortransceivers located in the directions where that scramble beamovershadows all other beams, about which of these transceivers will benext in line to communicate with the array using one of the secure databeams. In one embodiment of the present invention, the communicationchannel provided by the scramble beam is used to organize a “contentionperiod,” for transceivers located in the directions where that scramblebeam overshadows all other beams, about which of these transceivers willbe next in line to communicate with the array using one of the securedata beams. In one embodiment of the present invention, the scramblebeam organizes a contention period using any of the well-known methodsfor medium access control in multiple access communication channels,such as binary countdown (Tanenbaum, Computer Networks, p. 260).

In one embodiment of the present invention, the scramble beams compriseone or more difference patterns. In one embodiment of the presentinvention, the data signal beams comprise one or more sum patterns. Inone embodiment of the present invention, the method further comprisesthe step of adjusting the data beams and the scramble beams using beamsteering.

In one embodiment of the present invention, the method further comprisesthe steps of transmitting the data beams and transmitting the scramblebeams using one or more array antennas. In one embodiment of the presentinvention, the method further comprises the step of adjusting the databeams and the scramble beams using analytical array synthesistechniques. In one embodiment of the present invention, the methodfurther comprises the step of adjusting the data beams and the scramblebeams using iterative array synthesis techniques.

In one embodiment of the present invention, the data signal bits aredivided into two or more parts. For each part of the data signal, acorresponding scramble-beam direction is defined that is slightly awayfrom the direction of the data beam. Each part of the data signal isthen transmitted while the scramble beam has its central null steeredtowards a corresponding scramble-beam direction. The division of thedata signal must be such that an intelligible signal is obtained only ifall the data bits are received

In one embodiment of the present invention, the scramble beams haveapproximately constant amplitudes away from their central null, so thatthe total radiated power is approximately omni-directional.Constant-amplitude scramble beams are achieved by moving zeros far offthe Schelkunoff unit circle or by iterative methods.

In one embodiment of the present invention, the method further comprisesthe step of transmitting the data beams and the scramble beams using aplanar array antenna. In one embodiment of the present invention, themethod further comprises the step of transmitting the scramble beamsusing cosine and sine difference patterns.

In one embodiment of the present invention, the method further comprisesthe step of transmitting data and scramble beams with spatialdependencies that are indistinguishable away from the null of thescramble beam.

In one embodiment of the present invention, one or more of the securedata beams are scanned in angular increments over the area within rangeof the array using beam steering while transmitting at each angle aunique identification number that can be recorded by transceivers towhom the data beam is intelligible and returned to the array as a methodof identifying and locating the positions of transceivers within rangeof the array. In one embodiment of the present invention, theidentification numbers returned to the array are used to create andstore a table indicating the current positions and signal strengths ofall transceivers within range of the array. In one embodiment of thepresent invention, the transceivers are RFID tags. In one embodiment ofthe present invention, the transceivers are laptop computers. In oneembodiment of the present invention, the transceivers are cell phones.

Embodiments of the present invention are directed to methods forsecurely and simultaneously transmitting multiple data streams overradio waves comprising transmitting one or more data beam comprisingmultiple data signals with multiple radiation patterns, transmitting oneor more scramble beam comprising multiple signals from which the datacannot be inferred with radiation patterns, and adjusting the data beamand the scramble beams such that a scramble beam overshadows each databeam in all but selected directions.

The present invention is directed to a method for preventingunauthorized transmitters from gaining access to a wireless network. Oneembodiment is a method comprising the steps of receiving signals with anantenna system that has more than one output port, computing a firstoutput with a first receiving pattern that has its main beam pointedtowards an authorized transmitter, computing a second output with asecond receiving pattern that has a null in the direction of theauthorized transmitter and is larger in magnitude than the firstreceiving pattern away from the authorized transmitter, computing theenergy of the first and second outputs, and passing the informationcontained in the first output on to the network only if the energy ofthe first output is larger than the energy of the second output.

In one embodiment of the present invention, the second receiving patternis computed from a combination of one or more difference patterns. Inone embodiment of the present invention, the first receiving pattern iscomputed from a combination of one or more sum patterns.

In one embodiment of the present invention, the step of computing thefirst and second outputs uses beam steering. In one embodiment of thepresent invention, the step of receiving signals uses one or more arrayantennas.

In one embodiment of the present invention, the step of computing thefirst and second outputs uses analytical array synthesis techniques. Inone embodiment of the present invention, the step of computing the firstand second outputs uses iterative array synthesis techniques.

In one embodiment of the present invention, the step of receivingsignals uses a planar array antenna. In one embodiment of the presentinvention, the step of computing the first and second outputs usescosine and sine difference patterns.

In one embodiment of the present invention, the step of receivingsignals uses a ring array antenna. In one embodiment of the presentinvention, the step of receiving signals uses a reflector antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a linear array with 18 elements having elementspacing equal to half a wave length.

FIG. 2 illustrates a sum pattern of the array in FIG. 1 evaluated atθ=90°. The array coefficients shown above the plot can be used for bothtransmission and reception.

FIG. 3 illustrates a difference pattern of the array in FIG. 1 evaluatedat θ=90°. The array coefficients shown above the plot can be used forboth transmission and reception.

FIG. 4 illustrates the sum and difference patterns of the 18-elementarray evaluated at θ=90°. The two sets of array coefficients shown abovethe plot can be used for both transmission and reception.

FIG. 5 illustrates the bit-error rate at θ=90° for the 18-element array.

FIG. 6 illustrates a linear array with 4 elements having element spacingequal to 10 cm.

FIG. 7 illustrates data and scramble beams of the array in FIG. 6evaluated at θ=90°. The array elements are patch antennas and both setsof excitation coefficients are shown above the plot.

FIG. 8 illustrates one data beam and two scramble beams of the array inFIG. 6 evaluated at θ=90°. The array elements are patch antennas andboth sets of excitation coefficients are shown above the plot.

FIG. 9 illustrates a linear array with 8 elements having element spacingequal to 6.25 cm.

FIG. 10 illustrates one set of data and scramble beams of the array inFIG. 9 evaluated at θ=90°. The array elements are z-directed dipoles andboth sets of excitation coefficients are shown above the plot.

FIG. 11 illustrates another set of data and scramble beams of the arrayin FIG. 9 evaluated at θ=90°. The array elements are z-directed dipolesand both sets of excitation coefficients are shown above the plot.

FIG. 12 illustrates a square planar array with 324 elements.

FIG. 13 illustrates a mapping of the array coefficients for the sumpattern of the 324-element planar array. The array coefficients can beused for both transmission and reception.

FIG. 14 illustrates a 3-D mapping of the array sum pattern correspondingto the array coefficients in FIG. 13.

FIG. 15 illustrates a mapping of the array coefficients for the cosinedifference pattern of the 324-element planar array. The arraycoefficients can be used for both transmission and reception.

FIG. 16 illustrates a 3-D mapping of the array difference patterncorresponding to the array coefficients in FIG. 15.

FIG. 17 illustrates a circular ring array.

FIG. 18 is a perspective view of a reflector antenna with its receivingdevice at the focal point.

FIG. 19 illustrates the secure perimeter set up.

FIG. 20 illustrates two sum patterns for a 4-element array forsecure-perimeter applications. Both beams use the array coefficientsshown on top. The two beams are steered in different directions.

FIG. 21 illustrates a polar plot of the two sum patterns for a 4-elementarray for secure-perimeter applications.

FIG. 22 is a block diagram of a secure receiving array in the presenceof transmitters, having a guard processor to determine which datastreams come from legitimate transmitters and may pass to the network.

FIG. 23 illustrates a secure 18-element array in the presence of twotransmitters T1 and T2 located in the x-y plane.

FIG. 24 illustrates an 18-element linear array operating at a centerfrequency around 2.4 GHz, with authorized and unauthorized transmitterspresent.

FIG. 25 illustrates the sum and difference beams pointing towards T1, anauthorized transmitter.

FIG. 26 illustrates the sum and difference beams pointing towards T2, anauthorized transmitter.

FIG. 27 illustrates the instantaneous phase of the sum output, theextracted bits, and the sum and difference energies for channel #1 whenthe excitation amplitude of T3 is 0 dB.

FIG. 28 illustrates the instantaneous phase of the sum output, theextracted bits, and the sum and difference energies for channel #1 whenthe excitation amplitude of T3 is 40 dB.

FIG. 29 illustrates the instantaneous phase of the sum output, theextracted bits, and the sum and difference energies for channel #1 whenthe excitation amplitude of T3 is 60 dB.

FIG. 30 illustrates the instantaneous phase of the sum output, theextracted bits, and the sum and difference energies for channel #2 whenthe excitation amplitude of T3 is 0 dB.

FIG. 31 illustrates the instantaneous phase of the sum output, theextracted bits, and the sum and difference energies for channel #2 whenthe excitation amplitude of T3 is 20 dB.

DETAILED DESCRIPTION OF THE INVENTION

The invention is first described for the transmission process, andsubsequently for the receiving process.

The Transmission Process

A physical layer of security may be obtained with the present inventionby feeding each element of an array with a total signal that is obtainedby adding at least one scramble signal to the data signal. Two types ofarray patterns widely used in radar applications are of particularinterest to the present invention: (1) the sum pattern and (2) thedifference pattern.

The system for providing a secure radio link consists of an array ofantennas, electronics, and processors that control the electronics.Assume, for example, that it is desirable to use the array to transmitdata to a particular client. The time signal is represented by a(t); andrepresents the required time signal that must be fed to the arrayelements to facilitate the data transmission. The time signal a(t)depends on the chosen modulation and coding techniques and on thetransfer functions of the antenna elements. The present invention worksfor any modulation and coding techniques and for any set of arrayelements. The term “data signal” is used to describe a(t) because thedata for the intended client is embedded in a(t). For purposes ofillustration, we will first consider linear arrays such as the18-element array in FIG. 1.

Linear Arrays

FIG. 1 is a graph of a linear array with 18 elements having elementspacing of half of a wavelength. In standard operation, one would feedarray element #p with a signal of the form:T _(p)(t)=A _(p) a(t−α _(p))where A_(p) is the excitation coefficient and α_(p) is the time delay(p=1, 2, . . . , N). Generally, one could feed each array element withtime functions that have different time dependence to compensate forarray imperfections, end-element effects, or array elements differences.Such adjustments would be well known and straightforward to thoseworking in this area. Therefore, for the purpose of this example, it isassumed that the time dependence of each input signal is the same (theamplitudes and time delays are different). The array excitationcoefficients and time delays (A_(p) and α_(p), p=1, 2, . . . , N, whereN is the number of elements) are determined by standard methods toachieve a desired radiation pattern of the array that adapts to itsenvironment.

FIG. 2 is a graph of a sum pattern of the 18 element array shown in FIG.1, evaluated at θ=90°. A typical array radiation pattern and theassociated excitation coefficients are shown in FIG. 2 for the18-element linear array shown in FIG. 1. The element spacing is equal tohalf a wavelength. All time delays are zero, so the array operates inbroadside mode. Since all the excitation coefficients have the samesign, the array radiates a sum pattern, which is characterized by a mainbeam and some side lobes that are below a certain level (−55 dB in theembodiment shown in FIG. 2). Beam steering can be achieved by assigningnonzero values to the time delays, which results in the complexexcitation coefficients A_(p) exp(i2πf₀α_(p)) when the exp(−i2πf₀t) timedependence is suppressed and f₀ is the frequency at which the sumpattern is evaluated. These issues are well known to those working inthis area.

In addition to the sum pattern, a difference pattern may be broadcast. Adifference pattern and the associated excitation coefficients are shownin FIG. 3 for the 18-element array shown in FIG. 1. The differencepattern has a deep null in the center that is surrounded by two steeppeaks. The term “difference pattern” is used because half of theexcitation coefficients are positive and the other half negative. Thetime delays in FIG. 3 are all zero. As seen for the sum pattern, beamsteering can be achieved by assigning nonzero values to the time delays.

A physical layer of security is obtained with the present invention byfeeding each element with a total signal that is obtained by adding atleast one scramble signal to the data signal. In the case of onescramble signal b(t), the total input signal to array element #p is:T _(p)(t)=A _(p) a(t−α _(p))+B _(p) b(t−β _(p))where B_(p) (p=1, 2, . . . N) are the excitation coefficients and β_(p)(p=1, 2, . . . N) are time delays for the scramble signal. This simplearrangement of signals creates a physical layer of security when thedata excitation coefficients A_(p) (p=1, 2, . . . N) produce a sumpattern and the scramble excitation coefficients B_(p) (p=1, 2, . . . N)produce a difference pattern. To steer the sum and difference beams inthe same direction, one simply sets α_(p)=β_(p).

To see how a physical layer of security is provided, the sum anddifference patterns of FIGS. 2 and 3 are plotted together and shown inFIG. 4. The data signal a(t) is transmitted through the sum pattern andthe scramble signal is transmitted through the difference pattern. Anobserver located at the nulls of the difference pattern will receiveonly the data signal. Conversely, an observer located at nulls of thesum pattern will receive only the scramble signal. In most locations,however, there are no nulls and an observer would receive a weighted sumof the data and scramble signals. The weights are simply the sum anddifference patterns at that particular location.

As shown by way of example in FIG. 4, it is evident everywhere outsidethe narrow angular region 87°<φ<93° that the difference pattern isgreater in magnitude than the sum pattern. Hence, an observer located atthe angle φ will receive the following signals:

-   -   φ=90°: the pure data signal a(t).    -   87°<φ<93°: a weighted sum of data and scramble signals in which        the weight for the data signal is greatest.    -   0°<φ<87° or 93°<φ<180°: a weighted sum of data and scramble        signals in which the weight for the scramble signal is greatest.

With any radio receiver, if the signal to noise ratio falls below acertain level, the receiver cannot demodulate the signal properly torecover the information. Consequently, in this example, only observersin the narrow angular region 87°<φ<93° will understand the data signal.Additionally, the angular region in which the data signal can beunderstood is likely even narrower due to noise.

The physical layer of security achievable with an 18 element array (asshown in FIG. 1) and with the present invention is demonstrated. Thedata signal is shielded by a scramble signal that makes the transmittedsignal unintelligible in every direction except in a narrow angularregion around the direction of the intended receiver. As a consequence,the area requiring monitoring for possible eavesdroppers issignificantly reduced when this extra layer of physical security isadded. In fact, an eavesdropper must be almost in the line-of-sightbetween the transmitter and receiver and may thus disturb theelectromagnetic field in a way that is readily detectable.

The sum and difference patterns in FIG. 4 are almost parallel in theregion where they attain values above −40 dB, except near the centralnull of the difference pattern. Therefore, the spatial dependence of thedata and scramble signals are almost the same in this region. If signalsbelow −40 dB are below the noise floor, the data and scramble signalscannot be separated based on the spatial dependence of the patterns,even if an eavesdropper uses an array receiver. The data and scramblebeam coefficients can be determined with well-known methods to achievepatterns that are parallel to an arbitrarily high degree. One can ensurethat the sidelobe region is below the noise floor by choosing the arraylarge enough, or by adding noise sources.

Example 1

For the purposes of this example, assume that the 18-element lineararray in FIG. 1 operates at a center frequency of approximately 2.4 GHz.The element spacing is 6.25 cm and the distance between the end elementsis 106.25 cm. The array is fed with sum and difference signals thatresult in the array patterns that are plotted as shown in FIG. 4 at 2.4GHz. The input signal is broadband, so the array-element distance equalsprecisely half a wavelength only at the center frequency. Withdifferential phase shift keying implemented using 1000 periods of a 2.4GHz sine wave per bit, a simulation, without noise, reveals that thesignal is intelligible in the φ=90° plane only in the angular region87°<φ<93°, as shown in FIG. 5.

In the simulation, the bits are computed in the following way: First theinstantaneous phase of the transmitted signal is computed with theHilbert transform. Second, for each bit transmission period (1000periods of the 2.4 GHz sine wave) a center phase is defined as the valueof the instantaneous phase at the center of that transmission period.Finally, the received bit is set equal to one if the difference betweenthe current and previous center phase is larger than 180°. The actualnumerical value of the bit-error-rate in FIG. 5 outside the region87°<φ<93° results from the discrepancy between the “Transmitted bits”and the “Scramble bits.” To achieve high security, one may choose thescramble bits from a random sequence.

All the zeros of the difference pattern, except the center zero atφ=90°, have been moved off the Schelkunoff unit circle to ensure thatthe magnitude of the difference pattern is larger than the magnitude ofthe sum pattern throughout the angular regions 0°<φ<87° and 93°<φ<180°.As a result, the excitation coefficients for the difference pattern arenot asymmetric around the center of the array. The sum of theseexcitation coefficients, however, still equals zero. By increasing thepower of the scramble signal, one may reduce the width of the angularregion in which the data signal dominates.

The array pattern resulting from any excitation of this array issymmetric around the x axis. The vector radiation pattern of thetransmitted electromagnetic field, however, equals the array patternmultiplied by the element vector radiation pattern, assuming that allelements have the same radiation pattern. Hence, if the elements haveradiation patterns with beams that peak at θ=90°, the transmittedelectromagnetic field will have a main beam in the direction (θ,φ)=(90°, 90°). Dipoles and patch antennas have such radiationcharacteristics. Beam steering of both sum and difference patterns maybe achieved with standard methods.

A Method for Reducing the Width of the Data Beam

FIG. 6 shows a four-element array operating around 900 MHz, with elementspacing=10 cm and total array length=30 cm. The intended client islocated near the (θ, φ)=(90°, 90°) direction. For the purposes of thisexample, assume that the array elements are made of patch antennas withsin²(φ) radiation patterns in the forward direction and very lowradiation pattern in the backward direction. FIG. 7 shows the field ofthis array for forward directions 0<φ<180°. The patterns of the patchantennas ensure that the field in the backward directions 180°<φ<360° islow.

The width of the angular region of the data signal is reduced bydividing the data signal bits into two parts: the first part and thesecond part. The first part is transmitted while the scramble beam hasits central null steered slightly to one side of the direction of thedata beam. The second part of the data signal is transmitted while thescramble beam has its central null steered slightly to the other side ofthe direction of the data beam. The division of the data signal must besuch that an intelligible signal is obtained only when both the firstand second part of the data signal are received.

FIG. 8 shows how this method can be implemented with the four-elementarray in FIG. 6, with patch antennas. Scramble beam #1 is obtained bysteering the central null to φ=99°. Scramble beam #2 is obtained bysteering the central null to φ=81°. The excitation coefficients areprovided above the plot in FIG. 8. A receiver must then be located in avery narrow region (at most, 15° wide in this example) around φ=90° inorder to receive both the first and second part of the data signal.

In principle, there is no lower limit on the width of the data-signalregion obtainable with this method. One may even divide the data signalinto three or more parts and employ three or more scramble beams, aslong as the reception of all parts of the data beam is required toextract the data. This method of reducing the width of the data-signalregion works for the other types of antennas described below. Inparticular, it works for planar arrays if the two scramble-beams nullsare steered in orthogonal directions (planar arrays require two scramblebeams as explained below). Another way of reducing the width of thedata-signal region is to continuously vary the direction of the scramblebeam while the data signal is being transmitted. Another way of reducingthe width of the data-signal region is to increase the power of thescramble beam(s), and thereby move the scramble-beam shoulders above thepeak of the data beam.

A Constant-Level Scramble Beam

An array may be designed such that its radiated power isomni-directional while its data signal stays highly directional.Consider the 8-element array in FIG. 9 that consists of z-directeddipoles and operates at 2.4 GHz, with element spacing=6.25 cm and thetotal array length=50 cm. The intended clients are near the (θ, φ)=(90°,90°) direction.

Typical data and scramble beams for this array are shown in FIG. 10. Allthe zeros of the scramble beam, except the central one at φ=90°, aremoved radially off the Schelkunoff unit circle to points on a circle inthe complex plane of radius 1.06. (The theory related to the Schelkunoffunit circle is described in the reference “Antenna Theory and Design” byR. S. Elliot, IEEE Press, 2003.) As a result, the scramble beam has onlyone null (the central one) and stays above the data beam everywhereelse. Notice that the scramble beam “follows” the data beam closely, sothat the power of the scramble beam is extremely low away from a 60°angular region centered on φ=90°. Hence, the array provides littleenergy to communicate with the clients that are located outside this 60°angular region.

FIG. 11 shows a non-typical scramble beam that has all its zeros, exceptthe central one, located on a circle in the complex plane of radius1.46. This scramble beam has an almost constant amplitude away from thecentral zero at φ=90°. Hence, it may communicate with any client locatedaway from φ=90°. The excitation coefficients are provided above the plotin FIG. 11. All the excitation coefficients for the scramble beam inFIG. 11 are positive except the last one, which equals −1. The sum ofthese coefficients equals zero.

The array used in this section operates at 2.4 GHz. The method forcreating a reader with an omni-directional power pattern works for anyfrequency that results in electromagnetic wave propagation. Instead ofusing the Schelkunoff unit circle representation to achieve theconstant-level scramble signal, one can use the iterativearray-synthesis methods with appropriate cost functions. The iterativemethods can be used directly to achieve constant-level scramble beamsfor ring arrays and planar arrays.

Planar Assays

FIG. 12 shows a 324-element planar array, having 18 elements by 18elements. The element spacing is half a wavelength. FIG. 13 shows atypical set of sum excitation coefficients, and FIG. 14 shows thecorresponding array sum pattern. The array pattern is almost independentof φ and has a main beam in the broadside direction. Standard methodscan be used to steer the beam in any desired direction.

For planar arrays, the difference patterns with sharp nulls have cos(φ)or sin(φ) angular dependence. The φ independent difference patterns forplanar arrays result in a broadening of the angular regions in which thesignals are intelligible. FIG. 15 shows a set of difference excitationcoefficients with cos(φ) angular dependence, and FIG. 16 shows thecorresponding difference pattern.

The excitation coefficients for both the sum and difference patterns forthe planar array may be obtained with semi-analytical methods to achievedesired side lobe levels and main beam widths. Alternatively, theexcitation coefficients may be obtained with nonlinear optimizationtechniques. Indeed, the excitation coefficients shown in FIGS. 13 and 15were obtained with the MATLAB™ function FMINUNC, which minimizes auser-defined cost function. The cost function is designed to ensure thatthe side lobes are below a certain level for all φ.

The difference pattern as shown in FIG. 16 has a null for φ=90° andφ=270°. Hence, with one scramble signal a secure transmission cannot beachieved if an eavesdropper is located at any observation point withφ=90° or φ=270°. Therefore, at least two scramble signals are necessaryfor the planar array. The excitation coefficients and array pattern fora sin(φ) difference pattern may be obtained by rotating the plots inFIGS. 15 and 16 ninety degrees around the z axis. To achieve a securetransmission in the direction θ=0°, the cos(φ) or sin(φ) differencepatterns are combined and each array element is fed by the sum of threesignals. Array element #p is thus fed by the signal:T _(p)(t)=A _(p) a(t−α _(p))+B _(p) b(t−α _(p))+C _(p) c(t−χp)where B_(p) and C_(p) are the excitation coefficients, β_(p) and χ_(p)are time delays, and b(t) and c(t) are the scramble signals applied tothe cos(φ) and sin(φ) difference patterns, respectively (p=1, 2, . . .N). As before, A_(p) (p=1, 2, . . . N) are the excitation coefficientsand α_(p) (p=1, 2, . . . N) are the time delays for the data signal.With at least two independent scramble signals, one achieves a securetransmission that is only intelligible in a narrow region round θ=0°. Tosteer the sum and difference beams in the same direction, one simplysets α_(p)=β_(p)=χ_(p).

Other Antennas

Secure communication in accordance with the present invention can alsobe achieved with arrays that are neither linear nor planar. For example,a circular array, as shown in FIG. 17, may be used for providing 360°coverage. Secure transmission with a circular array type can be obtainedby combining sum and difference patterns obtained from standard theory.Similar security measures can be realized with reflector antennas (FIG.18) by applying the present invention to its feed, which is typically asmaller antenna located at the focal point. More generally, one may usethe theory developed for any antenna type array configuration to obtainsum and difference patterns that can be combined to achieve securetransmissions, in accordance with this invention.

For purposes of illustration, the examples in this discussion have beenlimited to sum and difference patterns because such patterns have beenstudied extensively in the radar literature. However, securetransmissions in accordance with this invention can be achieved with anycombination of array patterns in which one of the patterns, the scramblesignal pattern, has a null in the direction of the intended receiver andis larger in magnitude than the data signal pattern in directions whereeavesdroppers may be present.

In many applications, one antenna array must communicate with multipleusers simultaneously. The present invention allows multiple securetransmissions from one antenna to occur simultaneously. For example,assume that a planar array needs to communicate with two receivers. Thesignals to be transmitted are a₁(t) for receiver #1 and a₂(t) forreceiver #2. Element #p of the planar array is thus fed by the signal:

T_(p)(t) = A_(1p)a₁(t − α_(1 p)) + B_(1p)b₁(t − β_(1 p)) + C_(1 p)c₁(t − χ_(1p)) + A_(2p)a₂(t − α_(2 p)) + B_(2 p)b₂(t − β_(2 p)) + C_(2 p)c₂(t − χ_(2 p))where the quantities with index 1 are chosen to create a securetransmission in the direction of receiver #1, and the quantities withindex 2 are chosen to create a secure transmission in the direction ofreceiver #2. To minimize interference, one may choose the arrayexcitation coefficients such that the beams are as narrow as possible.With this arrangement of two data signals and four scramble signals, aneavesdropper will not receive an intelligible signal unless located in adirection close to either receiver # 1 or receiver #2. Moreover,receiver #1 will not be able to understand what is transmitted toreceiver #2, and receiver #2 will not be able to understand what istransmitted to receiver #1. This procedure may easily be extended tomore than two receivers. The beams can be steered toward receiver #1 andreceiver #2 by setting α_(1p)=β_(1p)=χ_(1p) and α_(2p)=β_(2p)=χ_(2p).

The difference patterns must be slightly broader than the sum patternsto achieve the physical layer of security. The numerical examplesprovided here have demonstrated that difference patterns can be designedto have beam widths that are just slightly broader than the beam widthsof the corresponding sum patterns. Hence, the beam width of the totalradiation pattern for the secure transmission is almost as narrow as thebeam width for the corresponding insecure transmission.

Secure Perimeter

In the examples above, the information was transmitted into a narrowangular region. Consider an application where the information istransmitted into a wide angular region where multiple receivers may bepresent. This scenario occurs when eavesdroppers are present outside abuilding in which all users are trusted. Everyone inside the building isallowed access to the information, whereas everyone outside the buildingis an eavesdropper.

The schematic in FIG. 19 shows how one can prevent eavesdroppers outsidea perimeter from accessing the information. FIG. 19 shows only a segmentof a perimeter. Security around a closed perimeter can be achieved byplacing several such arrays at points on that perimeter. In FIG. 19, thearray placed near the perimeter is driven by a data signal that containsthe information and a scramble signal. Both signals are broadcast with asum beam. By beam steering, the information is broadcast into the regionenclosed by the perimeter and the scramble signal is broadcast to theoutside.

FIG. 20 shows two sum beams for a 4-element array operating at 2.4 GHzwith 5.6 cm element spacing. The array coefficients are also shown inFIG. 20. FIG. 21 shows the same beams as a polar plot superimposed on aschematic of the perimeter. Beam 1 containing the data is stronger thanBeam 2 containing the scramble signal in a wide angular sector thatpoints into the region enclosed by the perimeter, and Beam 1 is weakerthan Beam 2 in a wide angular sector that points out of the regionenclosed by the perimeter. Hence, a user inside the perimeter gets theinformation, whereas an eavesdropper outside the perimeter gets thescramble signal. Reflections from walls and other barriers can be takeninto account by adjusting the signal amplitudes and directions of thetwo beams, as is well known by a person skilled in the art.

Steering Information to Different Users

As seen from FIG. 5, the signal transmitted by the 18 element array inFIG. 1 contains the data bits (called the “transmitted” bits in FIG. 5)in the narrow six-degree region around φ=90°. Outside this narrowregion, the transmitted signal contains the scramble bits. The data bitsare transmitted with the sum beam and the scramble bits are transmittedwith the difference beam.

Alternatively, one could encode the sum beam with a first data setintended for users in the narrow region and encode the difference beamwith a second data set intended for users outside this narrow region.For example, the data set encoded in the difference beam could containinformation on network status. More generally, one could employ multiplesum and multiple difference beams to transmit multiple data streams intouser-specified angular regions. In this way, multiple informationstreams could be steered in multiple directions. The steep lobes of thedifference beams centered on the null enable one to transmit data with asum beam into a very narrow angular region.

Precise Location of Transceivers

A secure beam obtained by combining a sum beam and one or moredifference beams as described above can be used to efficiently andaccurately determine the positions of transceivers within range. Thetransceivers are not required to transmit secure beams. For example, thetransceivers could be laptop computers in a wireless network or RFIDtags. Their positions are determined as follows:

A secure-beam antenna (for example, a base station in a wireless networkor an RFID reader) scans the angular regions in which transceivers maybe present. The bits transmitted by the sum beam are encoded withinformation about the current scan angle. The difference beams ensurethat the information encoded in the sum beam is intelligible only in anarrow angular region for each scan angle. A transceiver records thebits transmitted by the secure-beam antenna and at an appropriate timetransmits those bits along with a transceiver-id number back to thesecure-beam antenna (or to another receiver). In other words, thetransceiver sends back information about its own location obtained fromthe data in the incoming secure beam.

Depending on the width of the intelligible signal region and the scanangle increments, a transceiver may receive the intelligible informationin the sum beam at more than one scan angle. Since the width of theintelligible signal region can be made very small, a precise location ofthe transceivers may be obtained without having to perform anyprocessing steps. The secure-beam antenna can also determine and recordthe quality (i.e., loss) of the propagation paths associated with eachscan angle. Thus, a database can be generated with transceiver positionsand propagation-path quality for each transceiver at each scan angle.

For some RFID applications, the sum beam does not need to containscan-angle information. Instead, one can design the reader and tags suchthat the tags respond only to the signal in the sum beam and not to thesignal in the difference beam. Hence, by pointing the reader to acollection of tags, one excites only those tags that are located in thenarrow region that receive the signal encoded by the sum beam.

The Receiving Process

The secure radio receiver in FIG. 22 consists of a receiving array ofantennas that is connected to a network through a guard processor. Theguard processor is the main component of the invention and will bedescribed in greater detail below. Transmitters attempting to access thenetwork are also shown in FIG. 22. Not all of these transmitters areauthorized to access the network, and the guard processor preventsunauthorized transmitters from gaining access.

For purposes of illustration, assume that the array of antennas has Nelements and u_(p)(t) represents the output of element #p. The outputdepends on the transmitters, the chosen modulation and codingtechniques, and on the transfer functions of the array elements. Thepresent invention works for any modulation and coding techniques and forany set of array elements. If multiple transmitters broadcastsimultaneously, u_(p)(t) is a weighted and time shifted sum ofcontributions from each transmitter.

The individual element outputs are combined into a single array outputaccording to the following equation:

${U(t)} = {\sum\limits_{p = 1}^{N}{H_{p}{u_{p}( {t - t_{p}} )}}}$where H_(p) represents real receiving coefficients and t_(p) representstime shifts. The equation sums the individual element outputs from p=1to N. The output U(t) depends on H_(p) and t_(p). More generally, beforecomputing U(t), one could correct each u_(p)(t) for array imperfections,end-element effects, or array elements differences. One working in thisarea would be familiar with making such corrections. Therefore, in thisdescription, one may simply use the outputs u_(p)(t) to compute U(t).

According to the present invention, the guard processor employsdifferent sets of receiving coefficients H_(p) to compute differenttotal outputs U(t). By comparing the energy of the total outputs, theguard processor ensures that only authorized transmitters from certainselected directions gain access to the network, regardless of the signalstrengths. This process may be explained by example for linear arrayssuch as the 18-element array shown in FIG. 23.

Linear Arrays

Assume for this example that the transmitter T1 in FIG. 23 is authorizedto access the network, whereas T2 is not authorized to access thenetwork. Both transmitters are in the plane θ=90°. As is well known, instandard operation, one would form the total time-domain outputaccording to the following equation:

${{U_{A}(t)} = {\sum\limits_{p = 1}^{N}{A_{p}{u_{p}(t)}}}},$where A_(p) represents the receiving coefficients. Since T1 is at φ=90°,all of the time delays are zero. The receiving coefficients aredetermined by standard methods to achieve a desired receiving pattern.

A typical receiving pattern and the associated coefficients areillustrated in FIG. 2 for the 18-element linear array shown in FIG. 23.The element spacing is equal to half a wavelength. All time delays arezero, such that the array operates in broadside mode. Since all thereceiving coefficients have the same sign, the array receives with a sumpattern, which is characterized by a main beam and some side lobes thatare below a certain level (−55 dB in this case). The receiving beam maybe steered by assigning nonzero values to the time delays, as is wellknown to those working in this area. U_(A)(t) designates the sum output.

The receiving beam in FIG. 2 points towards T1, such that the arrayboosts signals coming from the direction of T1 and attenuates signalscoming from the direction of T2. For example, if T2 is at φ=42°, wherethe sum pattern is at 60 dB, the amplitude of the signal from T2 must be60 dB larger than the amplitude of the signal from T1, in order for thetwo transmitters to contribute equally to the output U_(A)(t). If thesignal from T2 is 70 dB greater than the signal from T1, for example,the output U_(A)(t) will approximately equal the signal from T2 andunauthorized transmitter T2 would gain access to the network.

To prevent unauthorized transmitter T2 from gaining access to thenetwork, a difference pattern, as shown in FIG. 3, may be employed. Thedifference pattern has a deep null in the center, surrounded by twosteep peaks. The term “difference pattern” originates from the fact thathalf of the receiving coefficients are positive and the other halfnegative. The time delays as shown in FIG. 3 are all zero. As discussedpreviously, beam steering can be achieved by assigning nonzero values tothe time delays.

All the zeros of the difference pattern, except the center zero atφ=90°, have been moved off the Schelkunoff unit circle to ensure thatthe magnitude of the difference pattern is larger than the magnitude ofthe sum pattern throughout the angular regions 0°<φ<87° and 93°<φ<180°.As a consequence, the receiving coefficients for the difference patternare not asymmetric around the center of the array. The sum of thesecoefficients, however, still equals zero.

The output related to the difference pattern is computed according tothe following equation:

${U_{B}(t)} = {\sum\limits_{p = 1}^{N}{B_{p}{u_{p}(t)}}}$where B_(p) represents the difference pattern coefficients. Thedifference pattern has a null in the direction of T1, so that U_(B)(t)will never equal the signal transmitted from T1. For the configurationshown in FIG. 23, the difference pattern is at −50 dB at T2's locationφ=42°. U_(B)(t) designates the difference output.

FIG. 4 shows both the sum and the difference patterns. The differencepattern is greater in magnitude than the sum pattern everywhere exceptin a narrow region around φ=90°. This observation indicates that theenergies of U_(A)(t) and U_(B)(t), measured during an appropriate timeinterval in which T1 transmits, can be used to determine if U_(A)(t)actually contains the information transmitted by T1. The energy ofU_(A)(t) and U_(B)(t) is noted over the selected time interval by E_(A)and E_(B), respectively. The MAC protocol ensures that any otherauthorized transmitter is silent when T1 transmits, so that the receiveronly has to discriminate between T1 and an unauthorized transmitter suchas T2. The guard processor performs the following comparison:

-   -   If E_(A)>E_(B), then the information in the sum output U_(A)(t)        equals the information transmitted by T1, and this information        is passed on to the network.    -   If E_(A)≦E_(B), then the sum output U_(A)(t) may contain the        information transmitted by T2, and access to the network is        denied.

Applying this approach to the configuration shown in FIG. 23, the sumpattern is 10 dB lower than the difference pattern at the location φ=42°of the unauthorized transmitter T2, and the contribution from T2 toU_(A)(t) is 10 dB weaker than the contribution from T2 to U_(B)(t).Because T1 does not contribute to U_(B)(t), the condition E_(A)>E_(B)can only be achieved if T1's contribution to U_(A)(t) is on the order of10 dB greater than T2's contribution. In this case, using theconfiguration shown in FIG. 23, T2's contribution to U_(A)(t) iscompletely overshadowed by T1's contribution, so U_(A)(t) contains onlythe information of T1 and access to the network may be granted.

According to this example, the sum pattern should be at least a coupleof dB below the difference pattern outside the region of the authorizedtransmitter. If the levels of the sum pattern and difference patternsare too close at certain angles, one may simply raise the level of thedifference pattern by increasing the magnitudes of the coefficientsB_(p).

Example 2

For this example, assume, in a complicated time-domain simulation, thatthe 18-element linear array in FIG. 23 operates at a center frequency ofapproximately 2.4 GHz. Then the element spacing is 6.25 cm and thedistance between the end elements is 106.25 cm. Three transmitters arepresent in the plane θ=90° as shown in FIG. 24. T1 and T2 are authorizedtransmitters, and T3 is an unauthorized transmitter. The transmittershave different excitation amplitudes and the signals exhibit 1/r decay.All of the transmitters broadcast simultaneously and the signals arecoded with differential phase shift keying. Each bit uses 1000 periodsof a 2.4 GHz sine wave.

When two authorized transmitters broadcast simultaneously, two sets ofsum and difference beams are necessary. These beams point towards T1 andT2, as shown in FIGS. 25 and 26. The sum output for channel #1 isdefined according to the following equation:

${U_{A\; 1}(t)} = {\sum\limits_{p = 1}^{N}{A_{p}{u_{p}( {t - t_{p\; 1}} )}}}$and the difference output for channel #1 is defined according to thefollowing equation:

${{U_{B\; 1}(t)} = {\sum\limits_{p = 1}^{N}{B_{p}{u_{p}( {t - t_{p\; 1}} )}}}},$where A_(p) and B_(p) represent the sum and difference receivingcoefficients, and t_(p1) represents time shifts that steer the beamstoward T1. The sum and difference outputs for channel #2 are definedsimilarly with time shifts t_(p2) that steer the beams towards T2.

As described above, the information is obtained from the sum outputs.The difference outputs are used in the energy comparison that determinesif access to the network should be granted. Each sum output receivescontributions from all three transmitters, whereas each differenceoutput receives contributions from only two of the three transmitters.For example, the difference output for channel #1 does not receive T1,according to the earlier discussion.

In the example, the bits transmitted by the three transmitters are:

${T\; 1{\text{:}\begin{bmatrix}1 & 1 & 0 & 1 & 1 & 0 & 0 & 0 & 0 & 1 & 0 & 1 & 0 & 0 & 1\end{bmatrix}}},{T\; 2{\text{:}\begin{bmatrix}1 & 0 & 1 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 1 & 0 & 1 & 1 & 0\end{bmatrix}}},{and},{T\; 3{{\text{:}\begin{bmatrix}1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1\end{bmatrix}}.}}$

The received bits are computed in the following way: First, theinstantaneous phase of the total array output is computed with theHilbert transform. Second, for each bit transmission period (1000periods of the 2.4 GHz sine wave) a center phase is defined as the valueof the instantaneous phase at the center of that transmission period.Finally, the received bit is set to one if the difference between thecurrent and previous center phase is larger than 180° and the receivedbit is set to zero if the difference between the current and previouscenter phase is smaller than 180°.

The excitation amplitudes for T1 and T2 are maintained at 0 dB and −26dB, respectively. First, consider channel #1. FIG. 27 shows theinstantaneous phase of the sum output, the extracted bits, and the sumand difference pattern energies when the excitation amplitude of T3equals 0 dB. The energy of the sum pattern output is 30 dB greater thanthe energy of the difference pattern output, so access to the network isgranted. The bits extracted from the sum pattern output are the bitstransmitted from T1.

The excitation amplitude of T3 is 40 dB in FIG. 28. The sum patternenergy is greater than the difference pattern energy, and the extractedbits are the bits sent by T1. It is the high directivity of the sumpattern beam that allows correct reception of the transmission from T1in the presence of the strong signal from T3.

The excitation amplitude of T3 is 60 dB in FIG. 29, and the sum patternenergy is less than the difference pattern energy, so access to thenetwork is denied. The extracted bits are those transmitted by theunauthorized transmitter T3. Hence, the unauthorized transmitter T3would have gained access to the network if the method of the presentinvention were not employed.

FIGS. 30 and 31 show the outputs of channel #2 for the cases where theexcitation amplitude of T3 equals 0 dB and 20 dB, respectively. Since T2is weaker than T1, access is granted only when the excitation amplitudeof T3 is 0 dB. As shown in FIG. 31, the correct bits transmitted from T2are extracted, but the sum pattern energy is less than the differencepattern energy and access is denied. The instantaneous phase is stronglyaffected by T3, however, so it is appropriate to deny access.

Planar Assays

The present invention may also be used with planar arrays such as the324-element array (18 elements by 18 elements) shown in FIG. 12. Theelement spacing is half of a wavelength. FIG. 13 shows a typical set ofsum receiving coefficients, and FIG. 14 shows the corresponding arraysum pattern. The array pattern is almost independent of φ and has a mainbeam in the broadside direction. Standard methods, as previously noted,can be used to steer the beam in any desired direction.

For planar arrays, the difference patterns with sharp nulls have cos(φ)or sin(φ) angular dependence. FIG. 15 shows a set of differencereceiving coefficients with cos(φ) angular dependence, and FIG. 16 showsthe corresponding difference pattern.

The receiving coefficients for both the sum and difference patterns forthe planar array may be obtained with semi-analytical methods to achieveprescribed side lobe levels and main beam widths. Alternatively, thecoefficients may be obtained with nonlinear optimization techniques. Thecoefficients as shown in FIGS. 13 and 15 were obtained with the MATLAB™function FMINUNC, which minimizes a user-defined cost function. The costfunction is designed to ensure that the side lobes are below a certainlevel for all φ.

The difference pattern shown in FIG. 16 has a null for φ=90° and φ=270°.With only one difference beam, unauthorized network access is notprevented for an unauthorized user located at any observation point withφ=90° or φ=270°. Therefore, at least two difference beams are used for aplanar array. The receiving coefficients and array pattern for a sin(φ)difference pattern are obtained by rotating the plots as shown in FIGS.15 and 16 ninety degrees around the z axis. Three total outputs may becalculated according to the following equations:

${{U_{A}(t)} = {\sum\limits_{p = 1}^{N}{A_{p}{u_{p}( {t - t_{p}} )}}}},{{U_{B}(t)} = {\sum\limits_{p = 1}^{N}{B_{p}{u_{p}( {t - t_{p}} )}}}},{and},{{U_{C}(t)} = {\sum\limits_{p = 1}^{N}{C_{p}{u_{p}( {t - {–t}_{p}} )}}}},$where A_(p) represents the receiving coefficients for the sum patternbeam and t_(p) represents the time delays that point the beam toward theauthorized transmitter with the desired signal. Moreover, B_(p) andC_(p) represent the receiving coefficients for the difference patternbeams with cos(φ) and sin(φ) difference patterns, respectively. Theequations sum the individual element outputs over the 2D grid of arrayelements. The amplitude of the difference pattern beams are adjusted toensure that the sum of the difference pattern beams has a highermagnitude than the sum pattern beam outside the direction of theauthorized transmitter.

E_(A), E_(B), and E_(C) represent the energies of U_(A)(t), U_(B)(t),and U_(C)(t), respectively, measured during an appropriate time intervalas previously discussed. The guard processor prevents an unauthorizedtransmitter from getting access to the network by following theprocedure:

-   -   If E_(A)>E_(B)+E_(C), then the information in the sum output        U_(A)(t) equals the information transmitted by the authorized        transmitter, and the information is passed on to the network.    -   If E_(A)≦E_(B)+E_(C), then the sum output U_(A)(t) may contain        the information transmitted by an unauthorized transmitter, and        access to the network is denied.

In this particular embodiment, the guard processor bases its decision ona comparison of E_(A) and E_(B)+E_(C). Alternatively, one could compareE_(A) with a different combination of E_(B) and E_(C), or with theenergy obtained by combining U_(B)(t) and U_(C)(t).

Secure communication in accordance with the present invention may alsobe achieved with arrays that are neither linear nor planar. For example,the circular ring array shown in FIG. 17 is useful for providing 360°coverage. For ring arrays, unauthorized transmitters can be preventedfrom accessing the network by comparing the energies received by sum anddifference patterns obtained from standard theory. Similar securitymeasures can be realized with reflector antennas as shown in FIG. 18 byapplying the present invention to its receiving device, which istypically a smaller antenna located at the focal point of the reflectorsurface. More generally, one may use the present invention for anyantenna type to obtain sum and difference patterns that can be combinedto achieve the secure reception.

The examples herein are confined to sum and difference patterns becausesuch patterns have been studied extensively in the radar literature. Thesecurity feature that prevents unauthorized transmitters from gainingaccess to the network can be achieved, however, with any combination ofarray patterns in which one of the patterns (the “difference pattern”)has a null in the direction of the authorized transmitter and is largerin magnitude than the other pattern (the “sum pattern”) in directionswhere unauthorized transmitters may be present.

The numerical simulation involving the 18-element linear arraydemonstrates that the present invention also works when the antennareceives signals simultaneously from several authorized transmitters.

The difference patterns must be slightly broader than the sum patternsto achieve the security feature. The numerical examples presented hereindemonstrate that difference patterns may be designed to have beam widthsthat are just slightly broader than the beam widths of the correspondingsum patterns.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made herein without departing from the invention asdefined by the appended claims. Moreover, the scope of the presentapplication is not intended to be limited to the particular embodimentsof the process, machine, manufacture, composition of matter, means,methods, and steps described in the specification. As one will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized. Accordingly, the appended claims areintended to include within their scope such processes, machines,manufacture, compositions of matter, means, methods, or steps.

1. A method for preventing unauthorized transmitters from gaining accessto a wireless network comprising the steps of: receiving signals with anantenna system that has more than one output port, computing a firstoutput with a first receiving pattern that has a main beam pointedtowards an authorized transmitter, computing a second output with asecond receiving pattern that has a null in the direction of saidauthorized transmitter and is larger in magnitude than said firstreceiving pattern, computing the energy of said first output, computingthe energy of said second output, and transmitting information containedin said first output to said network only if said energy of said firstoutput is larger than said energy of said second output.
 2. The methodof claim 1, wherein said second receiving pattern is computed from acombination of one or more difference patterns.
 3. The method of claim1, wherein said first receiving pattern is computed from a combinationof one or more sum patterns.
 4. The method of claim 1, wherein said stepof computing a first output and said step of computing a second outputeach comprises using beam steering.
 5. The method of claim 1, whereinsaid step of receiving signals further comprises using one or more arrayantennas.
 6. The method of claim 1, wherein said step of computing afirst output and said step of computing a second output each comprisesusing analytical array synthesis techniques.
 7. The method of claim 1,wherein said step of computing a first output and said step of computinga second output each comprises using iterative array synthesistechniques.
 8. The method of claim 1, wherein said step of receivingsignals comprises using a planar array antenna.
 9. The method of claim1, wherein said step of computing a second output comprises using cosineand sine difference patterns.
 10. The method of claim 1, wherein saidstep of receiving signals comprises using a ring array antenna.
 11. Themethod of claim 1, wherein said step of receiving signals comprisesusing a reflector antenna.